1. Field of the Invention
The present invention relates to an electrophotographic apparatus for performing image formation by using plural light beams, which apparatus records a test pattern to detect an abnormal state (or wrong state).
2. Related Background Art
In recent years, a so-called multibeam laser printer which performs image formation by using plural light beams, e.g., plural laser beams and obtains a desired image through an electrophotographic process has been studied.
FIG. 12 shows an example of such the multibeam laser printer, and FIGS. 13A to 13I show operation timing of the printer.
In FIG. 12, a laser printer 1 is connected to an external equipment 31 such as a computer or the like, and performs image formation on a recording paper under the control of the equipment 31. The external equipment 31 supplies various control signals and image information to a video controller 27, and the controller 27 outputs a video signal. A print control unit 26 is a control circuit for controlling each unit in the printer 1. When an RDY signal from the external equipment 31 becomes TRUE as shown in FIG. 13A, the video controller 27 sets a PRINT signal TRUE as shown in FIG. 13B. When the PRINT signal becomes TRUE, the print control unit 26 starts to drive a main motor 23 and a polygonal motor 14 as shown in FIGS. 13F and 13G. When the motor 23 is driven, a photosensitive drum 17, fixing rollers of a fixing unit 9 and paper discharge rollers 11 start rotation. Then, the print control unit 26 starts to control a light quantity of a semiconductor laser 13, and also sequentially performs high-voltage driving of a primary charger 19, a development unit 20 and a transfer charger 21.
When a time T1 elapses from a drive start of the polygonal motor 14 and thus rotation of the motor 14 becomes stable as shown in FIG. 13G, the print control unit 26 turns on a paper feed clutch 24 to drive a paper feed roller 5 as shown in FIG. 13H. Thus, a recording paper sheet 3 within a paper feed cassette 2 is fed toward resist rollers 6. At timing when the paper 3 reaches the rollers 6, the unit 26 outputs a VSREQ signal to the video controller 27 as shown in FIG. 13C, and also turns off the clutch 24 to stop driving the roller 5 as shown in FIG. 13H. On the other hand, after the controller 27 expands the image information sent from the external equipment 31 into a dot image and then completes preparation for outputting a VDO signal, the controller 27 confirms that the VSREQ signal in FIG. 13C is TRUE. Then, the controller 27 sets a VSYNC signal TRUE as shown in FIG. 13D. In synchronism with such an operation, after elapsing a time Tv as shown in FIG. 13E, the controller 27 starts to output the VDO signal as image data corresponding to one page.
At this time, the print control unit 26 turns on a resist roller clutch 25 after elapsing a time T3 from rise of the VSYNC signal as shown in FIG. 13I, and drives the resist rollers 6. The rollers 6 are driven for a time T4 as shown in FIG. 13I, i.e., until a trailing edge of the recording paper sheet 3 passes through the rollers 6. During the time T4, the print control unit 26 drives the semiconductor laser 13 according to the VDO signal sent from the video controller 27.
The semiconductor laser 13 comprises lasers A and B which emit two laser beams, i.e., laser beams A and B respectively. The print control unit 26 drives each laser according to each VDO signal. The two laser beams are reflected by a rotating polygonal mirror 15 and then inclined by a mirror 16 in a scanner unit 7, and the inclined beams are guided onto each scan path of the photosensitive drum 17. For example, it is assumed that odd-number lines on the drum 17 are scanned by the laser beam A, while even-number lines are scanned by the laser beam B. As above, when the two laser beams modulated by the respective VDO signals are simultaneously radiated onto the photosensitive drum 17, a latent image is formed on the drum 17 such that two lines are formed by each beam. By repeating such an operation, the latent image of one page is formed on the drum 17. A not-shown beam detector is provided on the scan paths of the laser beams A and B and out of an image formation area. The beam detector detects the beams A and B, and generates /BD1 signal and /BD2 signal respectively corresponding to the beams A and B. Modulation timing of the laser beams is controlled on the basis of these two /BD signals.
The latent image formed on the photosensitive drum 17 is developed by the development unit 20, and then a toner image is transferred onto the recording paper sheet 3 by the transfer charger 21. After the transfer terminates, the paper 3 is carried to the fixing unit 9, and the toner image is fixed to the paper 3. After then, the paper 3 is discharged outward by the paper discharge rollers 11. In case of continuously printing an image of next page, the print control unit 26 again sets the PRINT signal TRUE after elapsing a time T5 as shown in FIG. 13B, and performs the same control as in the printing of the first-page image.
As a test pattern data generation circuit for such the multibeam laser printer, for example, a circuit for generating longitudinal-line test pattern data in a two-beam laser printer will be explained. FIG. 14 shows a structure of this circuit, and FIGS. 15A to 15J show operation timing of this circuit.
Hereinafter, structure and operation of FIG. 14 will be explained. A mask signal generation timing setting register 101 is a register which stores therein timing (=counter value) for releasing a /MASK1 signal 124 and a /MASK2 signal 224 necessary in test printing and timing (=counter value) for generating these signals. A storage operation into the resister 101 is performed at the beginning of the test printing.
In FIG. 14, in order to obtain horizontal synchronism in the test printing, a /BD1 signal 120 has been inputted in a first phase sync oscillator 102 and a first main-scan counter 103.
When the /BD1 signal 120 becomes TRUE as shown in FIG. 15A, the first main-scan counter 103 is initially reset. Subsequently, the first phase sync oscillator 102 generates an image clock signal (CLK1 signal) 121 in synchronism with the /BD1 signal 120 as shown in FIG. 15B. The CLK1 signal 121 is inputted to the first main-scan counter 103 and also to a counter 106 for generating test pattern data. Since the counter 103 counts the number of clock pulses, a first main-scan counter value 122 increases as time elapses. By a first comparator 104, the value 122 is compared with a counter value 123 for releasing a mask set in the mask signal generation timing setting register 101. On the other hand, a value of the counter 106 at this time is kept "0", because a /writing inhibition signal 126 is TRUE and thus the counter 106 is continued to be cleared.
Subsequent to the /BD1 signal 120, a /BD2 signal 220 changes its state from FALSE to TRUE as shown in FIG. 15F. Thus, in the same manner as in the above first main-scan counter 103, a second main-scan counter 203 is reset, a second phase sync oscillator 202 generates a second image clock pulse signal (CLK2 signal) 221 as shown in FIG. 15G, and the counter 203 counts the number of clock pulses. Even in a second comparator 204, a mask release value 223 of the laser B and a second main-scan counter value 222 are compared with each other. As a result, while the value 222 is smaller than the value 223, the /MASK2 signal 224 is kept TRUE.
When the first main-scan counter value 122 reaches the mask release value, a mask of the laser A is released as shown in FIG. 15C, and the /MASK1 signal 124 is inputted to a gate 105.
At this time, when a /TOPE signal 125 being FALSE is inputted to the gate 105, the four-bit first counter 106 starts counting as shown in FIG. 15D. The respective bits counted by the counter 106 are managed as input values into an NAND gate 107 to generate a /TEST PATTERN1 signal 127. When the value of the first counter 106=Fh, the signal 127 becomes TRUE as shown in FIG. 15E.
Also, when the second main-scan counter value 222 reaches the mask release value, a /TEST PATTERN2 signal 227 is generated in the similar manner.
When the first main-scan counter value 122 reaches a mask generation value, the /TEST PATTERN1 signal 127 becomes FALSE. Similarly, the mask is generated for the laser B, and the writing is inhibited. Such a series of operations is repeated until the /TOPE signal 125 becomes TRUE. Thus, a longitudinal-line test pattern is printed on the paper sheet.
Subsequently, examples of abnormal (or wrong) states which are specific to the multibeam laser printer will be explained, and also problems of the above conventional structures will be indicated.